Notable characteristics

Rhodium is a hard silvery white and
durable metal that has a high reflectance. Rhodium metal
does not normally form an oxide, even when heated. Oxygen is absorbed
from the atmosphere
at the melting
point of rhodium, but on solidification the oxygen is released.
Rhodium has both a higher melting point and lower density than platinum. It is not attacked by
acids: it is completely
insoluble in nitric acid
and dissolves slightly in aqua regia. A
complete dissolution of rhodium in powder form is only obtained in
sulfuric
acid.

Applications

The primary use of this element is as an
alloying agent for hardening platinum and palladium. These alloys are
used in furnace windings, bushings for glass fiber production,
thermocouple
elements, electrodes
for aircraft spark plugs,
and laboratory crucibles. Other uses include:

Plated rhodium, made by electroplating or
evaporation, is extremely hard and is used for optical instruments.

This metal finds use in jewelry and for decorations. It
is electroplated on white gold and
platinum to give it a reflective white surface. This is known as
rhodium flashing in the jewelry business. It also may be used in
coating sterling
silver in order to strengthen the metal from tarnish, as a
result from the copper compound found in sterling
silver.

It is also a highly useful catalyst in a number of
industrial processes (notably it is used in the catalytic system of
automobile catalytic
converters and for catalytic carbonylation of methanol to
produce acetic acid
by the Monsanto
process). It is used to catalyse addition of hydrosilanes to a
double bond, a process important in manufacture of certain silicone
rubbers.

It is also used as a filter in mammography systems because
of the characteristic x-rays it produces.

It is also used in high quality pen surfaces due to its
high-resistance characteristics. These pens include Graf von
Faber-Castell which is rather less famous than Montblanc, but
produces very limited pens.

Occurrence

Normal mining

The industrial extraction of rhodium is
complex as the metal occurs in ores mixed with other metals such as
palladium, silver, platinum, and gold. It is found in platinum ores
and obtained free as a white inert metal which is very difficult to
fuse. Principal sources of this element are located in South
Africa, in river sands of the Ural
Mountains, in North and South America and also in the copper-nickel
sulfide mining area of the Sudbury,
Ontario region. Although the quantity at Sudbury is very small,
the large amount of nickel ore processed makes rhodium recovery
cost effective. The main exporter of rhodium is South Africa
(>80%) followed by Russia. The annual world production of this
element is only about 25 tons and there are very few rhodium
minerals. As of October
2007, rhodium cost approximately eight times more than gold, 450
times more than silver, and 27,250 times more than copper by
weight. Rhodium's typical historical price is about $1,000/troy oz,
but in recent years it has increased to about $9500/troy oz.

As a fission product

It is also possible to extract rhodium
from used
nuclear fuel, which contains rhodium (1 kg of the fission
products of 235U contain 13.3 grams of 103Rh). So as a typical
used fuel is 3% fission products by weight it will contain about
400 grams of rhodium per ton of used fuel. The longest lived
radioisotope of rhodium is 102mRh which has a half life of 2.9
years, while the ground state (102Rh)has a half life of 207
days.

Each kilo of fission rhodium will contain 6.62 ng
of 102Rh and 3.68 ng of 102mRh. As 102Rh decays by beta decay to
either 102Ru (80%) (some positron
emission will occur) or 102Pd (20%) (some gamma ray
photons with about 500 keV are generated) and the excited state
decays by beta decay (electron capture) to 102Ru (some gamma ray
photons with about 1 MeV are generated). If the fission occurs in
an instant then 13.3 grams of rhodium will contain 67.1 MBq (1.81
mCi) of 102Rh and 10.8 MBq (291 μCi) of 102mRh. As it is normal to
allow used nuclear fuel to stand for about five years before
reprocessing, much of this activity will decay away leaving 4.7 MBq
of 102Rh and 5.0 MBq of 102mRh. If the rhodium metal was then left
for 20 years after fission, then the 13.3 grams of rhodium metal
would contain 1.3 kBq of 102Rh and 500 kBq of 102mRh. At first
glance the rhodium might be adding to the resource value of
reprocessed fission waste, but the cost of the separation of the
rhodium from the other metals needs to be considered.

Isotopes

Naturally occurring rhodium is composed of only
one isotope, 103Rh. The
most stable radioisotopes are 101Rh
with a half-life of 3.3
years, 102Rh with a half-life of 207
days, 102mRh with a half-life of 2.9
years, and 99Rh with a half-life of 16.1 days. Twenty other
radioisotopes have been characterized with atomic
weights ranging from 92.926 u
(93Rh) to 116.925 u (117Rh). Most of these have half-lifes that are
less than an hour except 100Rh (half-life: 20.8 hours) and 105Rh
(half-life: 35.36 hours). There are also numerous meta states
with the most stable being 102mRh (0.141 MeV) with a half-life of
about 207 days and 101mRh (0.157 MeV) with a half-life of 4.34
days. See isotopes
of rhodium.

Precautions

Rhodium metal is, as a noble metal, inert.
However, when rhodium is chemically bound, it is reactive. Rhodium
compounds are not often encountered by most people and should be
considered to be toxic and
possibly carcinogenic. Lethal intake
(LD50) for rats is 12.6 mg/kg of rhodium chloride (RhCl3). Rhodium
compounds can strongly stain human skin. The element plays no
biological role in humans. If used in elemental form rather than as
compounds, the metal is harmless.

Symbolic uses

Rhodium has been used for honours, or to
symbolize wealth, when more commonly used metals such as silver,
gold, or platinum are deemed insufficient. In 1979 the
Guinness Book of World Records gave Paul
McCartney a rhodium-plated disc for being history's all-time
best-selling songwriter and recording artist. Guinness has also
noted items such as the world's "Most Expensive Pen" or "Most
Expensive Board Game" as containing rhodium.